1. Field of the Invention
The present invention relates to a rotary electrostatic encoder, and particularly relates to an absolute-type rotary electrostatic encoder that has a simple structure and is capable of obtaining an absolute angle.
2. Description of the Related Art
An example of a conventional electrostatic encoder is disclosed in Japanese Laid-open Patent Publication No. 2005-221472. The electrostatic encoder disclosed in this document is provided with a moving element that moves in relation to a fixed element, and has a structure capable of measuring the position of the moving element. In the electrostatic encoder, the fixed element is formed as a plate-shaped member having an induction electrode and a potential detection electrode, and the moving element on the other side is formed as a plate-shaped member having an electrode. The plate-shaped moving element is placed over the plate-shaped fixed element, and the moving element is provided so as to move along the fixed element in a state of being superposed on the fixed element. The sensor unit of the electrostatic encoder is thus formed. The sensor unit of the electrostatic encoder disclosed in Japanese Laid-open Patent Publication No. 2005-221472 has a thin profile and can be made in a small size.
A related conventional technique is the capacitive displacement encoder disclosed in PCT (WO) 2002-542476. This capacitive displacement encoder has a mechanism for sensing the rotation position of a disk-shaped moving body in relation to a disk-shaped static body. This mechanism is composed of at least one static element that is connected to a static body, and a moving element that is connected to a moving body in the vicinity of the static element. An electric field transmission device is provided for generating an electrostatic field. The electrostatic field is modulated by a capacitance change that occurs between the two elements in conjunction with relative rotational movement between the static element and the moving element. A processing circuit senses the modulated electrostatic field, and in response, determines the measured position of the moving body.
In PCT (WO) 2002-542476, the configuration shown in FIG. 2 detects the angle of a rotor. In this configuration, four phases of signals are fed to four electrodes provided 90 degrees apart on the stator side, thus driving the electrodes, and the signals are received by an eccentric electrode in the rotor. The angle of the rotor is detected by detecting the phase of the received signals. The configuration shown in FIG. 7 of the technique disclosed in PCT (WO) 2002-542476 has a three-plane structure formed by providing two stators with a rotor in between the two stators. An inner ring electrode and an outer ring electrode are formed concentrically on the opposing surface of one stator. In the outer ring electrode, four electrodes are formed 90 degrees apart. An inner ring electrode and an outer ring electrode are formed concentrically on the opposing surface of the other stator. Four-phase driving is performed for the four electrodes in the stator. The rotor positioned between the two stators is formed by a type of electrostatic shield, and the electrostatic shield has an eccentric electrode structure. A reception signal is read from the other stator described above. Since the electrostatic shield member of the rotor is eccentric, the phase of the reception signal varies in the range of 0 to 360 degrees according to rotation.
Another related conventional technique is the electrostatic capacitance encoder disclosed in Japanese Laid-open Patent Publication No. 08-233605. This electrostatic capacitance encoder has a structure in which the sensor unit is formed by two disk-shaped stators. The stators have a transmission electrode positioned in the center thereof, and twelve reception electrodes arranged at an interval on the periphery of the transmission electrode so that a three-phase reception signal is obtained. The rotor has a gear-shaped electrode that protrudes at an interval in four directions.
The capacitive displacement encoder disclosed in PCT (WO) 2002-542476 has the following drawbacks. Since the rotary sensor unit has a three-plane structure (sandwich structure) composed of two stators and one rotor, there are a large number of parts, and the structure is complex. A configuration is also adopted therein in which the electrostatic capacitance between the two stators is varied by the electrostatic shielding capability of the rotor. A ring-shaped electrode for providing a virtual ground must therefore be provided for each of the two stators positioned on both sides of the rotors. A buffer amplifier (indicated by reference numeral 182 of FIG. 7 of PCT (WO) 2002-542476) for potential control (virtual grounding) is also provided to the inner ring electrodes of the two stators. The electrode structure is therefore complex, and a circuit is required to stabilize the potential of the reception electrode.
In the electrostatic capacitance encoder disclosed in Japanese Laid-open Patent Publication No. 08-233605, an alternating-current power supply is connected between the transmission electrode and the ground, and a transmission signal is presented to the transmission electrode. However, since the ground is never connected to the electrodes of the rotor and the stator, it is unknown whether the ground actually has an effect.